1. Field of the Invention
This invention relates to an electrostatic recording material for a facsimile or a high speed electrostatic printer, such as a dielectric coated paper or a paper for transfer of an electrostatic image. More particularly, this invention relates to an electrostatic recording material for a recording layer of a dielectric coated paper in which an electrostatic latent image is directly formed on a dielectric recording layer by applying an electric change thereto, or of a paper for transfer of an electrostatic image in which an electrostatic latent image previously formed on an electrophotographic plate by an electrophotographic process is transferred to the paper.
2. Description of the Prior Art
Conventional recording papers comprise an electrically conductive layer and a dielectric layer superposed thereon on one surface of a base paper and an electrically conductive layer on the outer surface of the base paper. Materials used as the dielectric layer are highly insulating resins, e.g., organic solvent type resins such as silicone resins, epoxy resins, polyvinyl acetal resins, vinyl acetate resins, vinyl chloride, resins, and styrene-butadiene copolymers. These resins are generally dissolved in an organic solvent and coated on a base paper.
The dielectric layer must have a high surface inherent electrical resistance, e.g., higher than about 10.sup.10 .OMEGA., even under conditions of high temperature and high humidity, and, therefore, the above-described organic solvent type resins have heretofore been commonly utilized as a component of such dielectric layers.
However, the use of the above organic solvent type resins is disadvantageous due to the danger of ignition or explosion during coating thereof, and most of these organic solvents are toxic to humans. Therefore, the use of the organic solvent type resins requires specific equipment for the safety of the operators and for the recovery of the solvents used in order to prevent environmental pollution.
In addition, it is necessary to provide an under-coat layer as a barrier coating on a base paper prior to coating of a solution of an organic solvent type resin to prevent penetration of the solvent used into the paper.
In view of the above, attempts have been made to use water-soluble or -emulsifiable resins as a dielectric material in order to eliminate the above-described disadvantages associated with the use of organic solvent type resins.
Generally, water-soluble or -emulsifiable resins do not penetrate into base papers so that a barrier coating to prevent the dielectric coating material from penetrating into the base paper is not required.
However, there are also some problems in the use of water-soluble or -emulsifiable resins as a dielectric material, and, thus, these resins have not yet been practically used for producing dielectric coated materials.
U.S. Pat. No. 3,920,880 discloses an electrostatic recording material having a dielectric layer comprising a Copolymer A of (a) a carboxyl group-containing ethylenically unsaturated monomer, and (b) an ethylenic monomer or a conjugated diolefin and a Copolymer B of (a') an ethylenic monomer or conjugated diolefin, and (b') an amine solution of (i) a carboxyl group-containing unsaturated monomer and (ii) an ethylenic monomer or conjugated diolefin.
U.S. Pat. No. 3,920,880 teaches, as a representative example, a dielectric layer comprising ethyl acrylate, styrene, methyl methacrylate and methacrylic acid. While U.S. Pat. No. 3,920,880 discloses water-soluble resins, these water-soluble resins are always used in conjunction wth a water-insoluble resin.
One of the disadvantages of the water-soluble or -emulsifiable resins is that most of the resins are, in general, more hydrophilic than the organic solvent type resins, and, therefore, they are hygroscopic under high humidity conditions. Thus, deterioration of the charging characteristics of the dielectric layer results.
Another problem associated with the use of water-soluble or -emulsifiable resins is that surface active agents, e.g., emulsifying agents used in preparing a coating liquid of the resin, adversely affect the charging characteristics of a layer formed therefrom.